Navigational instrument

Abstract

A navigational system for spacecraft use utilizes two telescopes gimbaled on a common axis to measure the scalar angle between a star and the lunar limb. The common axis is the axis of rotation of a momentum wheel carrying a narrow mirror. When each tracker is aligned at its respective target, the mirror passes through the respective optical axis of the telescopes. Timing sensors record the passage to measure the angle and its complement. The phase locked loop used to maintain precision angular rotational rates of the momentum wheel is disclosed such that approximately 20 statistical measurements of the included angle per second can be made.

1.  A method of determining position fixes utilizing two optical receivers, comprising the steps of: a. positioning a first receiver on an axis of rotation into alignment with a first celestial body to receive radiation emitted therefrom;   b. positioning a second receiver on said axis of rotation into alignment with a second celestial body to receive radiation emitted therefrom;   c. developing a signal indicative of the scalar angle between the two optical receivers, said signal generated by measuring the time interval of constantly rotating means passing through the optical axis of said first receiver and then through the optical axis of said second receiver.   c. developing a signal indicative of the scalar angle between the two optical receivers, said signal generated by measuring the time interval of constantly rotating means passing through the optical axis of said first receiver and then through the optical axis of said second receiver.   
     
     
       2. The method of claim 1 wherein the complement of the scalar angle between the two receivers is determined by measuring the time interval of rotation of said constant rotation means through the optical axis of said second receiver and then through the optical axis of said first receiver. 
     
     
       3. The method of claim 1 wherein said time interval is determined by starting a digital counter when said constant rotation means passes through the optical axis of said first receiver and stopping said counter when said constant rotation means passes through the optical axis of said second receiver. 
     
     
       4. The method of claim 2 wherein said time interval is determined by starting a digital counter when said constant rotation means passes through the optical axis of said second receiver and stopping said counter when said constant rotation means passes through the optical axis of said first receiver. 
     
     
       5. An apparatus for determining position by obtaining a plurality of celestial fixes comprising: a. rotating means carrying a mirror on a portion thereof;   b. first and second optical means gimballed for movement on the axis of rotation of said rotating means and relative to said rotating means, said first optical means disposed to receive radiation emitted from a first celestial body and said second optical means disposed to receive radiation emitted from a second celestial body;   c. a first timing sensor disposed relative to said first optical means, said first optical means having means to direct radiation received from said first celestial body along a first optical path such that when said mirror on said rotating means crosses said first path, radiation will be directed on to said first timing sensor;   d. a second timing sensor disposed relative to said second optical means, said second optical means having means to direct radiation received from said second celestial body along a second optical path such that when said mirror on said rotating means crosses said second path, radiation will be directed on to said second timing sensor.   
     
     
       6. The apparatus of claim 5 wherein said rotating means is a momentum wheel rotating at constant speed and further including means to vary the rotational speed of said momentum wheel. 
     
     
       7. The apparatus of claim 6 further including an encoded disc rotating at the same speed as said momentum wheel, means to pick-off the code on said disc as it rotates, said code comprising a sine track and a cosine track and means responsive to said pick-off means to regulate the speed of said momentum wheel. 
     
     
       8. The apparatus of claim 7 wherein the means to regulate the speed of said momentum wheel includes a phase locked speed control loop coupled to said means to vary the rotational speed of said momentum wheel, said phase locked speed control loop having means to generate a carrier frequency, means to modulate said carrier frequency by said sine and cosine tracks respectively to allow phase measurement of the instantaneous position of said momentum wheel, means to sum said modulated signals, and comparator means to compare said summed signal with a signal representing the commanded angular velocity of said momentum wheel. 
     
     
       9. The apparatus of claim 5 wherein said first and second optical means comprise a pair of optical telescopes and further including means to rotate said telescopes about the axis of rotation of said rotating means. 
     
     
       10. The apparatus of claim 9 wherein said means to rotate said telescopes includes first motor means to rotate the first telescope and second motor means to rotate the second telescope independent of the rotation of said first telescope. 
     
     
       11. The apparatus of claim 10 further including first and second sensor means, each receiving emitted radiation from the respective celestial body each telescope is pointed at, each sensor developing pointing error signals to maintain said respective telescope aligned with the respective celestial body.